Abstract

Traditional, near surface geochemical techniques have been effective in mineral discovery but to explore in deeper transported cover (>10 m) and find new resources, as demanded by a growing global population, new and improved exploration methods are required. As mineral exploration transitions from exploring in residual outcropping terrains into deeper covered terrains, geochemical signatures are diluted and the ability to successfully discern subtle geochemical signatures is essential. At the ‘geochemically-blind’ North Miitel Ni deposit in Western Australia, we compared passive soil-gas hydrocarbons and weakly extracted elements from soil (10 – 20 cm depth) with both proving successful. The primary mineralization is >200 m below the surface, but a weak zone of secondary Ni enrichment occurs in the saprolite at 15 – 20 m depth. This enriched zone is covered by 10 – 15 m of transported cover. Minimum hypergeometric probability (MHP) statistics were used to evaluate the near surface geochemical signatures of c. 100 samples from three traverses over the mineralization. Soil samples from 10–20 cm were subjected to distilled water, 0.1 M cold hydroxylamine hydrochloride and aqua regia extractions. The water-extractable concentrations of Ni, Co, Mo, Sb, and Sn were successful in identifying mineralization (MHP <1%, type II error). The hydrocarbons, 2-methylbutane, pentane and 1-pentene were also successful (MHP <1%) at identifying the zone of mineralization using the Amplified Geochemical Imaging (AGI) passive soil-gas collectors. Of the techniques used, the water extraction performed the best using MHP classified accuracy to identify mineralization. The passive soil-gas data were the second most effective, and superior to the stronger partial extractions using hydroxylamine and aqua regia that did not identify the mineralized zone (MHP>>1%). The water extraction and the passive soil-gas showed a greater degree of variability than the stronger extractions. The results indicate that depth of sampling, interaction with organic carbon and potential mechanisms of metal migration greatly influence the geochemical anomaly near the surface. These mechanisms are evident as hydromorphic dispersion at depth, with potential capillarity and gaseous migration up through the profile above the water table. Integrating an understanding of metal migration mechanisms, genesis and evolution of target and pathfinder compounds (particularly hydrocarbons) related to deposit types will improve future exploration. Extending into much deeper cover (>20 m), the viability of passive soil-gas methods may become more relevant and warrant further study for mineral exploration.

Traditional, near surface geochemical techniques have been effective in residual and shallow cover terrains to find ore deposits, but to explore deeper and find new resources as demanded by a growing global population, new and improved exploration methods are required. Where the exotic cover is thick (e.g. >10 m), geochemical signatures from weathering ore deposits are diluted or not detected by conventional digestions; it is therefore essential to be able to measure subtle geochemical signatures. A potential improvement to strong partial extractions (e.g. aqua regia) that are widely employed by industry for near surface exploration, is weaker, partial extractions (e.g. deionized water). Another alternative is to use passive soil-gas collectors to investigate gas mobilized geochemical signatures. Both of these aim to identify subtle geochemical signatures from weathering ore deposits.

Recently, rapid mobility of (base and precious) metals was observed through sand cover, where 2 m of vertical metal migration occurred in <1 year in a pit experiment by Anand et al. (2014). This metal migration was exclusively detected with the use of a deionized water extraction. To further test metal-migration mechanisms, a field study site was selected that exhibited geochemically-blind mineralization to common surface geochemical tests (e.g. aqua regia) and had >10 m of transported cover. The North Miitel Ni deposit fits these criteria and was previously well characterized (Lintern et al. 2013; Noble et al. 2013a,b).

Near surface passive gas geochemistry for mineral exploration has been tested in the past (Kristiansson & Malmqvist 1987; Ball et al. 1990; Klusman 1993; Polito et al. 2002), but has not been widely studied compared with other sample media and certainly not widely adopted as a robust exploration tool for industry. Mineral exploration research has focused on light and abundant compounds like CO2 (Lovell et al. 1983; Polito et al. 2002), light hydrocarbons (C1–C6; Mulshaw 1996) and sulphur-bearing gases such as COS and CS2 (Oakes & Hale 1987), whereas even fewer studies have investigated metals and metalloids in soil-gas (Pauwels et al. 1999; Cao et al. 2009, 2010; Noble et al. 2013b). However, the integration of the organic and inorganic geochemistry data for near surface exploration has been rarely done. The lack of integration can be partly attributed to the paucity of published, peer-reviewed results and partly due to commercial sensitivity around the techniques. Both inorganic partial extractions and soil gas analyses have suffered from the lack of rigorous, external assessment that retards the ability of this science to progress. It is not ethical to encourage industry to use such invalidated or poorly understood methods, and so more unbiased studies are essential.

Previously, a number of commercial sampling devices such as Gore-sorber®, SDP (Soil Desorption Pyrolysis), SGH (Soil Gas Hydrocarbons) and OreHound GOCC® were used to collect soil-gas signatures over and around ore deposits. More recently, the Gore-sorber® technology has been further adapted for mineral exploration (Amplified Geochemical Imaging LLC, AGI), although to date, there are no peer-reviewed publications to rigorously assess the AGI passive soil-gas method.

Partial extraction soil geochemistry has been more widely reported than soil-gas studies in the literature and more widely adopted by industry for mineral exploration, although limited success has been reported from applications in arid terrains such as parts of Australia. A number of studies using partial soil extractions of metals have shown improved contrast and some ability to explore through cover (Seneshen 1997; Bajc 1998; Cohen et al. 1998; Xueqiu 1998; Gray et al. 1999; Kelley et al. 2003; Cameron et al. 2004; Eppinger et al. 2013). However, like soil-gases, partial extraction geochemistry has suffered from a lack of transparency and evaluation that has reduced uptake and application by industry. For instance, a number of proprietary soil and soil-gas exploration methods require the user to submit location data along with the samples, and although this practice is beneficial for the analytical service providers, it is unusual with respect to industry practice and standard geochemical testing. This strongly impacts on the further development of these techniques as authenticity and impartiality of the data are questioned. Therefore, with little external validation of the technique or statistical rigor, these techniques continue to be viewed with skepticism. In addition, numerous comparison studies are generally anecdotal with no formal quantitative comparison (Gray et al. 1999; Williams & Gunn 2002; Cameron et al. 2004; Mann et al. 2005). However, the application of minimum hypergeometric probability (MHP) statistics, although not routinely applied yet, has shown to be highly effective for the quantitative comparison of exploration techniques in orientation surveys (Stanley & Noble 2007, 2008; Noble & Stanley 2009).

The North Miitel Ni sulphide deposit was previously assessed by a number of partial extraction and passive soil-gas techniques and MHP was used to evaluate resulting anomalies (Noble et al. 2013a,b). Previous studies showed ore weathering was minimal with limited saprolite development and dispersion, and that the near surface environment was essentially ‘blind’ to the underlying ore from a number of techniques including soil sampling and analysis that was subject to aqua regia, four acid or Mobile Metal Ion (MMI) extractions, as well as Eucalyptus vegetation and leaf litter sampling and analysis (Brand 2005; Lintern et al. 2013; Noble et al. 2013a,b). Thus, there was no clear surface anomalism over the nickel sulphide ore with 10 – 15 m of cover using conventional soil extractions or biogeochemical techniques (Lintern et al. 2013; Noble et al. 2013a). Groundwater and regolith geochemical composition at depth successfully identified mineralization (Noble et al. 2013a); however, as drilling is commonly required to obtain such samples (particularly in arid Australia), the use of these technique have limited applicability. Passive gas collectors of metals and hydrocarbons (SGH technique from Actlabs and modified OreHound) revealed a subtle nickel concentration anomaly (Noble et al. 2013b). This signature could not be replicated when studying the adjacent soil chemistry (at the same depth and location as the collector) and this inferred a possible upward gaseous migration mechanism.

The blind North Miitel deposit was ideal to assess the techniques outlined in this paper. Hence, in this study, we have revisited this site to test if other soil-gas and partial extraction techniques offer more potential to explore for ‘blind’ deposits under cover. In this paper, we provide evidence for subtle soil and soil-gas geochemical anomalies and demonstrate that these weakly bound/mobile geochemical signatures can be used to effectively explore through transported cover (c. 15 m) where previous techniques had failed.

Study area and setting

The undeveloped North Miitel deposit, as well as the associated regolith, geological, biotic and climatic settings have all been previously described in detail (Lintern et al. 2013; Noble et al. 2013a). The site is located between Norseman to the south and Kambalda to the north about 600 km east of Perth, Western Australia. The study area is on the northern edge of Lake Zot, a saline playa (Fig. 1). This semi-arid area is a eucalypt dominated open woodland, with average annual rainfall of 289 mm and average maximum and minimum temperatures of 24.7 and 10.5°C, respectively (Norseman weather station; Bureau of Meteorology 2016).

The study site is located in an Archaean greenstone belt (2.7 Ga) of the Yilgarn Craton, on the eastern flank of the Widgiemooltha granite intrusive dome (Fig. 2). The greenstones are steeply dipping, mafic and ultramafic volcanic, felsic and clastic rocks with minor interflow sulphidic sediments and dolerites and are metamorphosed with dominant greenschist and amphibolite facies (Mincor Resources Ltd 2007).

Regional and local geology of the North Miitel area (GSWA 2004), and general regolith cross section at North Miitel along the 6506500N line. The Tertiary/Archean unconformity is represented by a thicker line (not to scale). Eastings (m) and northings (m) shown are GDA 94, Zone 51. A and B designation show the general location of the cross section on the local geology (from Noble et al. 2013a).

Elevated landforms are commonly comprised of mafic and ultramafic rocks, whereas sediment and felsic volcanic terrains occupy the lower landforms (Fig. 2). The North Miitel primary Ni mineralization is 200 – 400 m deep and is hosted within komatiites that are in direct contact with the basalts. The contact between the basalts and the mineralization has a NNW strike and dips steeply to the east (Fig. 2). The mineralization is characterized by abundant pyrrhotite, pentlandite, and minor pyrite and chalcopyrite in massive, matrix and disseminated sulphides (Mincor Resources Ltd 2007). Trace metal composition of the North Miitel ore is unknown. Studies on other komatiites in the region (Miitel and Mariners) show Co, Cu, Pt, Pd and As are understood to be associated with the primary Ni ore (Le Vaillant et al. 2015). It is likely that significantly smaller pods of mineralization occur closer (25 – 100 m) to the surface in profile as these are observed in the similar Miitel deposit to the south (Le Vaillant et al. 2015), and interpreted as a result of thrusting of the footwall basalts over the sulphide ore body (Stone & Archibald 2004). A minor zone of supergene enrichment is present at the weathering front. The supergene enrichment is thin (<2 m) and is weakly dispersed with an orientation to the west of 200 – 400 m metres based on regolith profile geochemistry (Noble et al. 2013a).

The supergene Ni enrichment at c. 17 – 20 m depth is towards the base of a thin ferruginous residual saprolite cover. This saprolite is <3 m in thickness. Overlying the residuum is a 10 – 15 m thick sequence of Quaternary aeolian sands and lacustrine/alluvial clays that comprise the Transported cover 10 – 80 m labelled in Figure 2 (inset cross section). The near surface soils are predominantly composed of quartz, kaolinite and Fe oxide (Noble et al. 2013a). The upper 30 – 50 cm thick sand contains a pedogenic carbonate and clay layer with an abrupt interface extending below this depth for another 2 m. The relief of the study area is generally low, with a small dip (c. 4 m) towards the eastern end of the sampled area. Red-brown fine silts and sands dominated by sheet flow in depositional zones are dominant. The water table, found 10 – 15 m below the surface, is commonly related to the saprolite horizon present over the mineralization and to alluvial clays observed near the eastern edge of the study site. Groundwater is saline (TDS >40 000 mg/L), acidic and oxidized which is common for the surface aquifer in this region (Gray 2001; Carey et al. 2003).

Methods

Soil-gas samples

The AGI collectors were installed according to the guidelines supplied with the collectors (AGI 2016). The passive soil-gas collectors were deployed into a 1.5 cm diameter hole at c. 50 – 60 cm depth using the installation kit provided and a battery operated drill with a 40 cm masonry drill bit. The drill hole was located in the bottom of the small 10 – 20 cm deep excavation pit used to collect a soil sample. The AGI samples were inserted into the base of the drilled hole and the hole was back filled and compacted. A total of 104 AGI sampling devices were installed in mid-October, 2015 (Fig. 3) and retrieved 63 days later in mid-December 2015. Three samples were lost in the field due to animals pulling out the collectors from the strings at the surface.

Aerial photo with the location of AGI passive soil-gas samples and shallow soil samples with the vertical projection of the mineralization. Additional sample locations from Noble et al. 2013a that were sampled in 2006 and reanalysed in this study are also shown. The northern most traverse (light purple crosses) was subject to additional extractions. The passive gas and soil samples from the 2015 study were collected at 5 m intervals over mineralization and 20 – 50 m intervals on either side of the mineralization.

The AGI passive soil-gas samplers have an ePTFE membrane (expanded polytetrafluoroethylene) and contain proprietary adsorbents for C2–C20 range of hydrocarbons, carbon dioxide, and several sulphur compounds.

Laboratory analysis

All of the field samples and trip blanks were shipped to AGI laboratories in Delaware, USA. The samples were analysed by thermal desorption, gas chromatography-mass spectrometry (Agilent 6890N GC-MS) for 86 compounds. Details of mineralization location and sample location were supplied on request to AGI as part of their testing of the technique in a new environmental setting. Following QA/QC (documented below) 48 compounds were deemed useable based on their concentration levels (Table 1) and data reported as raw numbers (ng/L) and with some additional interpretation from AGI. The compounds removed were not significantly different from field, trip and analytical duplicates using canonical variance analysis. This is also routinely reported with the data and plotted (Fig. 4).

General groups and specific compounds with the number of carbon molecules measured using AGI in the North Miitel study

Soil samples

Prior to installing the passive soil-gas collectors, c. 500 g of soil from 10 to 20 cm below the surface were collected. This was directly above where the AGI devices were installed (a further 40 cm below). The soil was dried and split. One split was sieved to −250 µm for use in the partial extraction analyses and another – 2 mm split was analysed for pH and electrical conductivity.

Laboratory analysis

Three separate partial extractions of the soils were conducted with all samples analysed for a multi-element suite of c. 40 elements using ICP-OES (Perkin Elmer Optima 7300DV) and ICPMS (Perkin Elmer Nexion 300Q) by LabWest Pty Ltd, Perth, Australia. The three extractions were:

Deionised water leach (LabWest PL02). Soil samples of 4.5 ± 0.1 g mass were weighed into a 50 ml polypropylene centrifuge tube with 45 ml of MilliQ water and shaken for 24 h. The solution was then centrifuged for 10 min (4500 rpm), and the supernatant was decanted into test tubes for analysis. The detection limits are reported in Table 2. This extraction was used to determine the concentrations of soluble elements in the soils.

Aqua regia digestion (LabWest PL05s). Soil samples (2.5 g) were subjected to an aqua regia digestion with a mixture of 3:1 concentrated HCl: HNO3 and analysed. The detection limits are reported in Table 2. This was a similar analysis to that conducted on the surface soils published by Noble et al. (2013a). The slight differences relate to the mass of soil and volume of solution, time of digestion and temperature used with different commercial laboratories. This difference may influence recovery, but the aqua regia extractions reported here were similar and are not expected to influence the extracted element chemistry greatly, maintaining the trends for element comparison.

Extraction techniques showing the range and detection limits for elements analysed in samples from the North Miitel study site. Concentrations are in µg/kg

Partial extractions were not applied to all samples for comparison because of cost constraints. All soil samples (104 from three traverses; Fig. 3) were subject to the water extraction, whereas only one of the three traverses (30 samples) was subject to the hydroxylamine hydrochloride and aqua regia extractions. The pH and electrical conductivity of 1:5 w/w soil to water ratio slurries were measured at CSIRO laboratories, Kensington for all samples collected in this study.

In addition to soils collected as part of this study, soil samples from a single traverse (34 samples) from earlier research (Noble et al. 2013a) were subject to the water extraction to see if this subtle geochemical signature had been overlooked in earlier research.

Quality control and data treatment

A previously analysed standard soil (GLG302-2 from GeoStats Pty Ltd) was incorporated into all geochemistry analyses as a standard replicate). Laboratory duplicates were analysed at a rate of 1 per 20 samples (n = 5 per extraction test). Percent relative standard difference was determined on the standard soil to be acceptable if it was <10% for the aqua regia, <15% for hydroxylamine extractions and <20% for the water leach as variation tends to increase as the partial extraction gets weaker and many of the analytes are near or below detection limits. The relative standard difference (RSD) error was calculated as a percentage for all compounds using method and field duplicate samples:The same criteria were applied to the standard soil and method duplicates. Field duplicates were included to evaluate variation for both the soil and soil-gas analyses at two locations. Field duplicate samples were placed 1 m apart. For soil extractions, Mo and Sb varied more than other elements in a single duplicate test using the water extraction (<30% RSD); all other data fit the stricter criteria above. The Sb and Mo duplicate that showed greater variation was in the mineralized zone and as such significant variation is not unreasonable within 1 m. The result of Mo and Sb were considered to be a real response and included for interpretation.

Soil-gas data regularly fluctuates (Klusman 1993) and requires additional QA/QC. The AGI technique used its own QA/QC documented as ‘fitness for use’ analysis with basic statistics, signal-to-noise comparison with response of field and QC sample classes based on raw data that is fully reported to the client. Instrument blanks, method blanks, inventory blanks (retained samplers assigned to the project that do not leave the laboratory) and trip blanks were used to assess error in the AGI workflow. Noisy compound variables and low-signal variables were removed, along with any sample that showed evidence of damage or contamination. Any compound for which the mean response of survey samples is not significantly (i.e. c. 2 times) greater than the response of inventory, instrument, and method blank data was removed (greyed out compounds in Table 2).

Canonical variates analysis (CVA) was used to determine if defined sample subsets, field samples and QC samples could be distinguished (Fig. 4). The results provide a comparison of field samples and blank (QC) sample population to show that survey field data are distinct from blank signatures and that no evidence of field or laboratory contamination is evident in field sample data.

Elemental results below detection were plotted at half the detection limit. In order to assess the predictive accuracy of the orientation survey with the various elements and extraction methods, minimum hypergeometric probability (MHP) statistics were used with a t-test significance at the MHP to assess contrast (Lintern 2007; Stanley & Noble 2007, 2008). Results <5% (Type II error) are commonly regarded as giving successful predictions (Stanley & Noble 2007, 2008), nonetheless, to enhance our confidence in the data, only results <1% were considered successful.

Principal Component Analysis (PCA) was used to better understand the groupings of elements, particularly those that were determined to predict the mineralized zone. For the soil chemistry, a subset of elements (c. 25) was selected as some elements were consistently below detection limits and others did not show any obvious variation (e.g. Cd, In, Pd, Pt, Re) or did not provide any valuable information (e.g. most REEs were not taken into account as they all varied similarly). Based on a Shiparo-Wilk test, most remaining elements showed a non- normal distribution, therefore, a Box-Cox transform was applied to the variables prior to PCA using the IoGAS software (Reflex v6.1).

Results

Soil-gas hydrocarbons and sulphur gases

The results from passive soil-gas collectors at North Miitel suggested that some hydrocarbons are anomalous over the mineralization in comparison with background areas. It is unclear whether these hydrocarbons are mobilized from weathering ore and moving from the deeper regolith, from the bedrock moving to the upper part of the profile or vary in response to the influence of microbial processes near the surface. Microbial processes generate CH4, but very rarely generate > C2 (dominantly thermogenic) hydrocarbons (Klusman 1993). Minimum Hypergeometric Probabilities (Table 3; Fig. 5; MHP <1%) indicated some compounds successfully predicted the ore zone. Other compounds such as ethane (C2H6) and carbonyl sulphide (COS) form more dispersed and lower contrast surface expressions adjacent to the mineralization. Although these compounds may provide a vectoring tool to some extent, these compounds do not predict the mineralized zone using the MHP assumed vertical migration model. The MHP evaluates the mineralized zone based on a priori understanding of which samples should be considered anomalous. It does not assess halos or adjacent samples that are anomalous. It is used for assessment of orientation studies and not for true exploration targeting. Carbonyl sulphide (with an MHP of 55.7%) does not predict the mineralized zone although it does have elevated concentrations adjacent to mineralization (Fig. 5). Ethane is similar (MHP 15.83% at a threshold of 49 ng). The pristane/phytane (Pr/Ph) value was tested as a potential vector, but was not successful in predicting the mineralization (MHP 5.44%).

Soil-gas results from the three traverses at North Miitel. Passive gas samples over mineralization are shown as red dots. All results are in nanograms except the pristine/phytane value which is unit-less. *indicates the MHP threshold significantly predicts the mineralized zone (MHP < 1%). MHPs are calculated for each compound for all three traverses collectively to provide one threshold value, hence some traverses individually do not look as well predicted as results in Table 3 suggest.

However, some soil-gases are effective. 2-methylbutane and pentane statistically predict the mineralized zone (MHP <0.31%; Table 3) and 1-pentene is even better (MHP 0.06%, Table 3; Fig. 5). The C5 compounds showed strong contrast based on the t-test statistic ≤ 0.008 (Table 3). Note that the following figures showing the MHP thresholds are determined for the whole data set (all three traverses combined) and not on individual traverses. This explains why some MHP thresholds look incorrect for an individual traverse.

Soil chemistry

The water extraction applied to the shallow soils showed that elevated concentrations of Ni and a number of elements (Co, Mo, Sb, Sn) successfully highlighted the presence of the mineralized zone. Anomalous Ni concentrations are correlated with mineralization and concentrations range from 36 to 625 µg/kg (Tables 2 and 3). MHP (Fig. 6; MHP commonly <0.01%) indicated that the results successfully predicted the ore zone. Arsenic was not a successful pathfinder predictor with MHP of 2% using the water extraction (Table 3). Elements not shown in Figure 6 were not statistically significant in the prediction of the mineralization (e.g. Cu).

Soil geochemical results from three traverses for selected elements extracted by water at North Miitel. Samples over mineralization are shown as red dots. Concentrations are in µg/kg. *indicates the MHP threshold significantly predicts the mineralized zone (MHP <1%). MHPs are calculated for each element for all three traverses collectively to provide one threshold value, hence some traverses individually do not look as well predicted as results in Table 3 suggest.

Concentrations of oxyanions, or weakly absorbed species, such as As, Mo and Sb were all anomalous over mineralization when extracted using water. Elevated Ni, Co, Cu and Sn concentrations were also indicative of the buried mineralization. Scandium was a non-traditional pathfinder element that was also observed in elevated concentrations over the mineralization (Tables 2 and 3). Antimony and to a lesser extent Mo proved exceptionally good pathfinder elements for the mineralized zone (Fig. 6). Antimony had an MHP threshold of 5 µg/kg, with a range of water-extracted concentrations from 0.6 to 10.6 µg/kg in the soils at North Miitel (Tables 2 and 3). The threshold is the concentration where the hypergeometric probability provides the minimum result and separates samples designated as anomalous or background.

The elements extracted using stronger partial extractions (aqua regia and hydroxylamine hydrochloride) did not show similar trends to the water extracted element results. Aqua regia extraction of soils did not produce any successful prediction of the mineralized zone using MHP tests, with the best results presented in Table 3 and Figure 7. Ni in the strong aqua regia and hydroxylamine hydrochloride shows greater concentrations to the west of the mineralization (Fig. 7) and is related to the underlying basalt lithology as reported in earlier studies (Noble et al. 2013a). Tin does not necessarily predict the mineralization with the stronger extraction, but certainly has greater concentrations over and adjacent to the mineralized zone.

Soil partial extraction geochemical results for Ni, Sb and Sn extracted by (A) aqua regia, (B) hydroxylamine hydrochloride and (C) water, from the northern-most traverse at North Miitel. Samples over mineralization are shown as red dots. MHP thresholds are also shown and *indicates the MHP threshold significantly predicts the mineralized zone (MHP <1%). Concentrations are in µg/kg.

Elevated concentrations of Mo (one point) and Bi (two points) were measured over the mineralization using soil subject to a 0.1 M hydroxylamine hydrochloride extraction, but other samples over mineralization showed background level concentrations. Antimony successfully predicted the mineralization (MHP = 0.007%), however, these results were much less convincing than the ones from the water extraction, with numerous values close to detection limit. The hydroxylamine hydrochloride extraction commonly targets amorphous Mn oxides and specific Fe oxides. In the North Miitel soils, these phases were not associated with any strong geochemical anomaly related to mineralization. Soil concentrations of elements extracted using the weakest (chemically least reactive), water partial extractions are commonly at least an order of magnitude lower than the ones measured after hydroxylamine hydrochloride extraction. Despite lower extraction capacity from the water partial extraction methods, the increase in elemental concentrations shows greater correlation with the mineralization.

A previous study (Noble et al. 2013a) has shown that soil chemistry of the organic-rich soil fraction sampled from 0 to 2 cm was not effective for exploration targeting. The soil collected during this previous study had been subject to aqua regia extraction. As the present results demonstrate that this method is not effective to detect subtle geochemical signatures, a water leach was applied to the samples collected in 2006. Results show that elements extracted by water in these organic-rich soils did not provide an indication of the underlying mineralization (Fig. 8) and the MHP assessment also supports that no significant results were found for the extracted elements (data not shown). It is possible that the organic signature deriving from the decay of litter overprinted the subtle geochemical signature observed at a slightly greater depth. In addition, the vegetation does not show any geochemical anomaly related to mineralization (Lintern et al. 2013). No additional drilling or activity had occurred at the site since 2006.

Soil geochemical results for selected elements extracted by water using the previously collected east–west-trending soil samples at North Miitel. Concentrations are in µg/kg. The location of these samples is shown in Figure 3.

Sulphur concentrations in the soil measured after partial extractions do not correlate (R2 <0.4) with major organic sulphur compounds (carbonyl sulphide, dimethyl sulphide, dimethyl disulphide, carbon disulphide) measured in the soil-gas. This discrepancy could be explained by the fact that S can be partitioned into various organic phases that were not specifically measured and may undergo changes through biogeochemical cycling. A weak correlation was evident between the aqua regia-extracted S and the hydroxylamine hydrochloride-extracted S (R2 = 0.7) that indicates a significant proportion of the S in the soil is associated with the Mn and Fe oxides, or alternatively, the S is evenly dispersed between Mn and Fe oxides and more resistive phases that are dissolved with aqua regia but not hydroxylamine hydrochloride; the former seems more likely. This could be determined by sequential extractions, but was not conducted as part of this study. The water soluble S was on average 60% of the aqua regia extracted S showing the water soluble fraction was dominant in the North Miitel soils. Sulphur does not show a spatial association with the mineralization.

The use of variable reduction with multiple regression techniques such PCA can improve interpretation of complex geochemical signatures. The use of PCA with water leached elemental data showed that PC1 and PC5 best separated the mineralized signatures (positive loadings for both components tended to be related to mineralized samples; Fig. 9), but the additional PC variables did not improve the ability to delineate mineralization any better than the individual elements. Principal Component 1 potentially shows higher loadings for water extractable elements that could be associated with the Ni-sulphide deposit, compared to the bulk regolith alkali and alkali-earth elements (Ca, Mg, Sr, K, Na).

PCA loadings of PC 1 (x-axis) v. PC 5 (y-axis) for elements extracted using the weak water leach at North Miitel.

The application of MHP with a data contrast test (t-test) provides an effective way to compare methods for exploration through cover at North Miitel. Figure 10 shows the water extracted Ni presents the most accurate value, whereas Sb shows the greatest contrast between mineralized and background concentrations. Therefore, it appears that both elements are very effective at targeting the mineralized zone, and the weakest partial leach (water) seems more suitable for numerous elements. Antimony extracted from the hydroxylamine hydrochloride and a range of hydrocarbon compounds presented the next best results for both accuracy in predicting the mineralized zone and contrast (Fig. 10). The aqua regia extracted elements showed the lowest potential for mineral exploration through cover at North Miitel.

Discussion

The North Miitel site was suitable to evaluate the effectiveness of novel or less proven techniques like the AGI passive soil-gas collectors and water extraction of soils for mineral exploration. Previous research found no clear surficial anomaly related to the buried North Miitel Ni mineralization (McDonald 1999; Brand 2005). More recent studies showed the presence of metals being dispersed at depth related to weathering and groundwater movement (Noble et al. 2013a), whereas the vegetation was not recycling metals from depth to the upper part of the soil profile, suggesting that the surface expression was effectively blind to traditional surface geochemical techniques (Lintern et al. 2013).

Whereas previous surface testing of soil and vegetation was largely unsuccessful (McDonald 1999; Brand 2005; Lintern et al. 2013; Noble et al. 2013a,b), a soil-gas Ni anomaly was observed using the OreHound GOCC® method. Although the soil-gas hydrocarbons (SGH from Actlabs) did not show a clear association with mineralization, some hydrocarbons showed anomalous results intermittently over or adjacent to mineralization. One near-surface signature that was not well investigated in the previous studies was the subtle Ni signature observed at the interface of sand to clay/carbonates at c. 40 cm depth (Noble et al. 2013b). If this anomaly can be observed, explained and reproduced, it offers a method to explore in truly challenging terrains that are commonly considered ‘blind’. Pedogenic carbonate horizon sampling has been successful for Au exploration in the southern part of Australia (Lintern 2001, 2015), but not for Ni (where it has not been extensively tested). This identified zone for sampling is just above this carbonate interface and corresponds to a zone of changed pH, partial pressure of O2, exchange capacity, texture and particle size distribution (Noble et al. 2013a). The AGI samplers and the previous metal soil-gas (OreHound® ­­­GOCC) samplers (Noble et al. 2013b) were also deployed near that depth. The soil-gas response near this carbonate horizon highlighted the presence of 2-Methylbutane, 1-Pentene and Pentane spatially associated with mineralized zone.

The current AGI hydrocarbon analytical suite does not report phenanthrene, dibenzothiophene and dibenzofuran, which is a present limitation as these compounds have an established link to base metal mineralization (Puttman et al. 1988, 1990; Bechtel et al. 2000). Many of these hydrocarbons have lower vapor pressures and high solubility, and are not expected to be highly concentrated on the AGI passive soil-gas samplers. Future development of these hydrocarbon analysis methods should ensure these compounds are added to the analytical suite. Benzofuran was analysed in this study, but was removed in the QAQC processing and could not be assessed. Evidence also exists that high grade S-rich ores biodegrade saturated and aromatic hydrocarbons (Bechtel et al. 1996) into smaller compounds that could include C3–C6 chains and branched alkanes and alkenes. Nonetheless, the influence of microbes on mobilization of metals and what organic signatures should be expected at the surface remain poorly understood.

Not all hydrocarbon studies successfully predicted mineralization and commonly faulting and fractured gas seeps showed the strongest gaseous response and skew potential for targeting of mineralization (Oakes & Hale 1987; Mulshaw 1996; Klusman 2009; Anand et al. 2016). The regolith cover at North Miitel is relatively shallow (10 – 15 m), permeable, and has limited seismicity influence (no fractures to the surface) which may explain the more consistent soil-gas signature detected at North Miitel in the present study as well as that of Noble et al. (2013b), in comparison with other sites where gaseous concentrations can be biased and increased in fracture zones with greater flux and unrelated to mineralization. Light hydrocarbon micro-seepage, in contrast to more rapid flow of gases along faults and fractures could form more diagnostic anomalies over mineral deposits. The AGI passive soil-gas collectors were used in this study to collect a micro-seepage signature. Whereas micro-seepage is a viable mechanism of organic and inorganic compound migration, the concept of microseepage would infer sample density could be reduced (greater sample spacing) as a broader and more consistent signature would be produced, but further research is required to assess the type and intensity of hydrocarbon and sulphur gas anomalies over blind mineral deposits and specific ore types.

Noble et al. (2013b) suggested that the Ni anomaly detected from the passive soil-gas collectors was not derived from the soil immediately surrounding the passive soil-gas collectors due to soil moisture leaching metals from the carbonate phase, but rather that the Ni anomaly derives from another source, such as metals bound to gas particles. The actual phase of Ni detected in the samples is proposed to be one or a combination of Ni nanoparticles, Ni carbonyl (Ni(CO)4) gas (Cody et al. 2000; Huber & Wächtershäuser 2006), and metal-S volatiles (Klusman 1993). The potential of S volatiles as a mechanism for metal migration may also explain why organic geochemical techniques such as the ones obtained from the AGI collectors and SGH (Noble et al. 2013b) show some unusual S soil-gas signatures around mineralization, but it is not congruent with the previous Ni-in-soil-gas anomaly of Noble et al. (2013b) or the water leached Ni anomaly in soils of this study (Fig. 6). This finding does not discount the gaseous migration mechanism proposed by Noble et al. (2103b), but clearly reemphasizes that the cycling of mobile Ni in the near surface environment remains poorly understood.

Surface geochemistry over cover of 10 – 20 m may still be beneficial for even deeper exploration, if used in the appropriate context and with a strong understanding of paleo-weathering fronts and traps for geochemical signatures. Previous studies revealed that these anomalies can form rapidly in transported cover (Anand & Robertson 2012; Anand 2016; Salama et al. 2016). In the present study, the recent metal migration was only detected in low (µg/kg) concentrations and only with very weak, partial extractions, which is similar to the experimental results of Anand et al. (2014) and Noble et al. (2011) that indicate rapid and transient metal migration through recent cover. The new results at North Miitel provide additional field-based evidence to support the concept of rapid and transient metal migration in cover.

The observed Mo, Sb and Sn soil anomalies are not well understood in relation to the North Miitel magmatic Ni mineralization. Limited research is published on non-traditional Ni komatiite pathfinder elements. Lesher et al. (2001) indicates chalcophile elements strongly partition with sulphides relative to silicate magmas, with possible associations or separation of first row transition metals, HREE v. LREE among others for a general study of komatiite ores. Antimony (bismuth) and Sn are chalcophile elements and may be associated with the sulphides in other ore deposits, but in this setting these elements are more likely associated with hydrothermal alteration or host rock chemistry. Antimony concentrations are reported by Le Vaillant et al. (2015) in selected samples from the Miitel mine drill core into alteration zones of the Mt Edwards footwall basalt, but Bi and Sn are not reported. This analysis targeted specific nickel arsenide grains in hydrothermal veins and as such the whole rock alteration geochemistry of these chalcophile elements in this adjacent setting also remains poorly understood. Strong evidence exists for the association of hydrothermal As with the magmatic Ni sulphides in the local area and in broader Western Australia. Selenium and Au may have some limited association with this hydrothermal overprint, too (Lesher & Keays 1984; Le Vaillant et al. 2015). Whereas As can be associated with the primary Ni ore formation event, in the North Miitel area this is unlikely (Le Vaillant et al. 2015).The more likely association of Sn and Mo (and W) may be associated with the adjacent sediments to the mineralization. The link of these unexpected pathfinder elements is speculative and requires further research.

One key finding of this study and those of previous studies at North Miitel, is that the signature is subtle and ‘blind’ to most conventional surface geochemical techniques. Results of this study indicate the presence of a near surface anomalous signature that is easily missed when using traditional sampling and analysis. In this area, weak partial extractions and trace soil-gas analyses clearly offer an advantage to explore through cover and one clear way forward for future exploration in cover.

Sample density is a key consideration in orientation studies and for future exploration application. Noble et al. (2013b) tested sample density changes at the North Miitel site by randomly removing samples to produce an even sample density as would be expected in a Greenfields exploration sampling program. In the study by Noble et al. (2013b) the results were still successful in identifying the mineralization at the less dense sample spacing (50 m compared to 25 m over mineralization). Although this was not applied in our study, many of the MHP results were orders of magnitude lower than the cut off value of 1% for significant results. As a result, if sample spacing was adjusted at random in this study (i.e. mineralized results removed), the pathfinder elements would remain effective (Table 3).

The highest concentrations for Ni within the upper few metres of regolith occurred at the interface of carbonate development and sand (Noble et al. 2013b). This is consistent with previous research (McDonald 1999) that also showed an increase of transition metals at c. 30 cm depth, just above the argillic horizon and the carbonate interface at North Miitel. This coincided with the textural change from sand to clay and supports other studies that report the B-horizon soils as being the best or strongest contrast sampling medium (Bajc 1998; Mann et al. 2005; van Geffen et al. 2012). Importantly, although the metal concentrations increased in this zone, it would not have clearly predicted mineralization as the contrast is minimal with stronger extractions.

At the clay/carbonate to sand interface, a mass influx of O2 and a large increase in plant, animal and microbial activity occurs compared to deeper in the profile (Buckman & Brady 1969). Nickel carbonyl (Ni(CO)4) was suggested as a potential gaseous migrating species that would decompose in contact with O2 that increases within the carbonate horizon due to proximity to the atmosphere and the increased partial pressure of O2 as well as enhanced biological activity (Noble et al. 2013b). This could explain the subtle increase in metal concentrations at the interface detected with weak partial extractions and passive soil-gas collectors. If pathfinder elements are brought up by gaseous mechanisms, anomalies in metal concentrations from soil samples should also be expected after weak partial extractions like water. Such anomalies in metal concentrations from soil samples were observed in the present study.

The AGI method is well developed for the sampling and analysis of soil-gas hydrocarbons and that the results were fully reported, i.e. specific compounds and concentrations were provided to the CSIRO research team. This full disclosure has not always occurred and this could be valuable in pursuing a better understanding of hydrocarbon signatures related to sulphide weathering. There are, however, limitations. The method requires a repeat sampling trip to collect the devices and the devices are somewhat prone to tampering from wildlife. In addition, the devices are shipped internationally for analysis (U.S. based) and the cost is more than common partial extractions of soil. In this study, water-extracted elements from soil showed stronger correlations with mineralization, but the soil-gases still presented greater correlations with mineralization than the more commonly used aqua regia soil extracted elements. While time and cost are key considerations for mineral exploration, there is potentially more upside to soil-gas analysis in deeper cover if we can match ore types to hydrocarbon pathfinders; this is major current knowledge gap. Modeled gaseous evolution of some sulphide minerals show H2S, COS, CS2, CH3,SH, (CH3)2S2 or SO2 or S2 should occur depending on Eh and pH conditions, although associated laboratory experiment produced CS2 and COS only (Taylor et al. 1982). This important study needs to be reproduced with current, more sensitive analysis as a first step to provide the link between near surface soil-gases and weathering ore deposits.

The ability of gases to migrate 100s of metres with potentially large fluxes in the order of 100 – 103 m per day (Etiope & Martinelli 2002) provides an unknown and exciting opportunity for mineral exploration through deeper and more recent transported.

Conclusions

The North Miitel Ni deposit in Western Australia has a weakly developed supergene Ni zone and, as such, a strong ‘conventional’ surface anomaly is unlikely to develop. Revisiting this well characterized ‘blind’ deposit has highlighted the ability of less conventional techniques to identify geochemical signatures. At North Miitel, we employed AGI passive soil-gas collectors along with water-leach, hydroxylamine hydrochloride and aqua regia extractions of soil to successfully predict the zone of mineralization. The application of minimum hypergeometric probabilities (MHP) and t-test contrast enables rigorous and impartial assessment of both hydrocarbon and inorganic geochemical responses. Water-leached Ni performed best, with other inorganic pathfinders (Mo, Sb, Co, Cu) also effective. The association of Mo and Sb with mineralization is unknown. With the 48 soil hydrocarbons measured, 2-methylbutane, pentane and 1-pentene presented greater abundances within the zone of mineralization. Carbonyl sulphide and ethane were anomalous directly over and adjacent to the mineralization. Sampling at the interface of the soil carbonate/clay with the overlying sand appears as the best geochemical exploration approach in this setting. The exact reasons for this interface to present a greater potential remain unclear, but could relate to the change in partial pressure of O2, pH, an increase in surface exchange capacity, and the proposed gas migration mechanisms discussed in this paper and other research.

Geochemical signatures from weathering ore deposits are diluted in transported cover and so it is essential to used weak partial extractions to dissolve less mineral substrates to recognize more subtle anomalous geochemical signatures. Two potential improvements to traditional near surface exploration are based on using passive soil-gas collectors and weak partial extractions as both of these aim to identify more recent active, geochemical signatures from weathering ore deposits. More research is required to understand the seasonal effects and spatial variability associated with weak partial extractions and also to comprehend the genesis and evolution of hydrocarbon signatures from weathering ore deposits. It is expected that more laboratory-based hydrocarbon studies will provide the fundamental understanding required to use soil-gas analysis routinely in the mineral exploration industry. Understanding vadose zone element migration mechanisms (particularly gases), target and pathfinder compounds related to deposit types and using new technology to measure these specific compounds, along with weak partial extractions like water, will provide a low impact method to explore through cover insuring future resource discovery.

Acknowledgements

We acknowledge the in-kind analytical support and comments from Mark Arnold of AGI, CSIRO internal reviewers (Sam Spinks and Nathan Reid) and GEEA external reviewers (Peter Winterburn and Matthew Leybourne) who improved this publication. The authors would also like to thank the staff at Mincor (Dave Chapman, Peter Mucelli and Richard Hatfield) who provided site access and sampling assistance, and Angelo Vartesi (Just Good Graphics).

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